In the case where vapor grown carbon fiber having developed carbon crystals is mixed with resin or other materials to form a composite, wettability of the carbon fiber to a matrix such as resin is a critical factor. When the vapor grown carbon fiber exhibits poor wettability to the matrix, the carbon fiber is difficult to disperse uniformly in the matrix and carbon fibers or vapor grown carbon fibers form aggregates isolated from one another in the matrix. Meanwhile, the surface of vapor grown carbon fiber in which carbon crystals are developed is inactive, and therefore the carbon fiber surface is not susceptible to a coupling agent or an additive.
In an attempt to modify the surface of carbon fiber or vapor grown carbon fiber, a variety of surface oxidation processes have been proposed. Typical examples of the processes are (1) acid treatment process making use of nitric acid or sulfuric acid, (2) air oxidation process, and (3) ozone oxidation process (see Japanese Laid-Open Patent Publication (kokai) No. 61-12967, Japanese Laid-Open Patent Publication (kokai) No. 2000-96429, and Japanese Laid-Open Patent Publication (kokai) No. 61-119767). In recent years, a process such as treating the surface of carbon fiber with fluorine in a plasma atmosphere has also been performed (see Japanese Laid-Open Patent Publication (kokai) No. 3-227325).
A functional group such as a carboxyl group, a carbonyl group, a hydroxyl group, or a similar group is introduced into the surface carbon layer of the carbon fiber after the oxidation treatment, and thus the carbon fiber surface becomes active. Therefore, the carbon fiber surface can be further modified by causing a coupling agent and the like to act on such a functional group in the surface (“Tanso Sen-i” (“Carbon fiber”) authored by Otani, Kindai Henshu Ltd. (1972)).